Conventionally, various types of substrate processing apparatuses have been known that perform a rinsing process for a substrate such as a semiconductor wafer (hereinafter a wafer) after the wafer is processed by the chemical solution. See, for example, Japanese Patent No. 3336952.
Generally, a substrate processing apparatus stores the chemical solution into a processing bath and immerses a plurality of wafers (for example, 50 wafers) into the chemical solution, thereby performing a chemical process for the wafers. Subsequently, the chemical solution in the processing bath is replaced with deionized water and the wafers are immersed into deionized water thereby performing a rinsing process of the wafers.
The substrate processing apparatus as disclosed in Japanese Patent No. 3336952 is equipped with a densitometer that detects a concentration of, for example, TOC (a total organic carbon) of a discharged solution discharged from the processing bath to a discharge line connected to the processing bath. The densitometer is configured to detect the concentration of the discharged solution discharged from the processing bath when the wafers are rinsed in a rinsing process. Generally, when the wafers are rinsed, the discharged solution from the processing bath is becoming cleaner gradually and the concentration of the discharged solution is becoming lower gradually. In particular, in the substrate processing apparatus as disclosed in Japanese Patent No. 3336952, the discharged solution is recovered and reused when the concentration of the discharged solution detected by the densitometer is lower than a predetermined concentration.
However, there is problem in the conventional substrate processing apparatus as disclosed in Japanese Patent No. 3336952 in that it is hard to recover an enough amount of discharged solution and, as a result, the efficiency of the apparatus is not satisfactory. Specifically, in the conventional substrate processing apparatus, the recovered solution tends to be relatively clean similar to deionized water. Thus, an amount of the discharged solution that can be recovered is relatively small because the concentration of the discharged solution from the processing bath does not become lower than a predetermined concentration unless a sufficient time is elapsed since the wafer rinsing process begins. Therefore, an amount of deionized water necessary for the chemical solution process or rinsing process of the wafers is increased and an overall operating cost such as a driving cost for refining deionized water is increased.